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# lect10_handout - Physics 102 Lecture 10 Faradays Law...

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Physics 102: Lecture 10, Slide 1 Faraday’s Law Physics 102: Lecture 10 Changing Magnetic Fields create Electric Fields

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Physics 102: Lecture 10, Slide 2 Last Two Lectures Magnetic fields Forces on moving charges and currents Torques on current loops Magnetic field due to Long straight wire Solenoid
Physics 102: Lecture 10, Slide 3 Motional EMF V A metal bar slides with velocity v on a track in a uniform B field Moving + charges in bar experience force down (RHR1) Electrical current driven clockwise! Moving bar acts like a battery (i.e. generates EMF)!! F q (Recall that e- actually move, opposite current) + q I

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Physics 102: Lecture 10, Slide 4 Faraday’s Law of Induction: “induced EMF” = rate of change of magnetic flux ? = − Φ Δ? = − Φ f − Φ i ? ? − ? ? The principle that unifies electricity and magnetism Key to many things in E&M Generating electricity Microphones, speakers, guitar pickups Amplifiers Computer disks and card readers
Physics 102: Lecture 10, Slide 5 First a preliminary: Magnetic Flux “Counts” number of field lines through loop. Uniform magnetic field, B , passes through a plane surface of area A . A Magnetic flux Φ = B A (Units Tm 2 = Wb) Magnetic flux Φ B A cos( φ ) φ is angle between normal and B B A φ normal B Note: The flux can be negative (if field lines go thru loop in opposite direction)

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Physics 102: Lecture 10, Slide 6 Preflight 10.7 Compare the flux through loops a and b. 1) Φ a > Φ b 2) Φ a < Φ b a b n n B Φ A = B A cos(0) = BA Φ B = B A cos(90) = 0 “more lines pass through its surface in that position.”
Physics 102: Lecture 10, Slide 7

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lect10_handout - Physics 102 Lecture 10 Faradays Law...

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